In recent years the inspection and analysis of goods that travel along conveyor, processing, manufacturing and assembly lines have become increasingly automated to match demands for greater production speed. Keeping up with such demands required quicker methods of performing quality control and quality assurance. Automated inspection systems perform a large portion of this work.
One type of detector used in such systems is a line scan camera. A line scan camera is a type of camera detector that can very quickly capture a single row of pixels of an object. The object is typically rotated while being scanned. The individual rows of pixels can then be assembled into a full image of the object. The assembled, flattened image looks as if the object were unwrapped. One advantage of this type of imaging system is the ease with which certain aspects of such images can be analyzed, especially by computer-aided, automated, and comparative techniques that obviate the need for manual object inspection. One application where line scan cameras are used is in the detection and sorting of produce. For example, U.S. Pat. No. 5,903,341 describes a produce grading and sorting system.
Another illustrative example is the use of line scan cameras in the manufacture of golf balls. Golf ball manufacturing typically involves a series of sequential processes performed at different processing stations. At many stages of assembly, processing, and handling, golf balls are automatically detected and imaged for comparison with paradigmatic golf ball images to ensure compliance within predetermined ranges of acceptable golf ball characteristics, including shape, color, shading, size, and appropriate printing of various indicia. Each such inspection process is monitored for quality assurance purposes. For example, U.S. Pat. No. 5,703,687 shows an automated inspection system that requires the addition of golf ball rotating equipment to the usual automated conveying equipment used to convey golf balls after an automated processing step. Another golf ball inspection system is disclosed in commonly owned U.S. Pat. No. 6,630,998 B1, which is incorporated herein by reference in its entirety. It has detectors and algorithms that allow for the automated visual testing of golf balls along each stage of the production line using a line scan camera.
Nevertheless, the spherical shape of golf balls and other round objects increases the difficulty of automated inspection of their three-dimensional surfaces because of distortion introduced into line scan images resulting from camera misalignments. When using a line scan camera, if the camera's scan line and the ball's axis of rotation are not parallel, a scanned image of the ball is distorted. Resulting images are slanted. For example, rectangular indicia that have right angles appear in the scanned images as slanted parallelograms when camera positioning is misaligned.
Likewise, if the line scan camera's scan line is not directly centered on the ball's axis of rotation, another image distortion can occur. Because the curvature of a sphere is perceptible when the scan line is focused on either side of the ball's axis of rotation, a straight vertical line drawn across the diameter of the sphere will appear curved instead of straight.
In a similar vein, if the line scan camera's scan line is not vertically centered on the ball's surface, the scanned images have yet another distortion. Because of the ball's curvature, when the scan line is not vertically centered on the ball's equatorial line as it rotates, its indicia appear to be vertically shorter in the image. Images of conforming golf ball's indicia appear shorter than comparable indicia in the paradigmatic images. Finally, a line scan camera can become unfocused, depending on the varying shapes and sizes of the scanned objects and on the varying positionings of the camera with respect to these objects.
Hence, there remains a need for an inspection system that can be calibrated to minimize distortions.